The present invention relates generally to a hydraulic control system for delivering fluid to one or more fluid actuated devices and more particularly to a hydraulic control system for controlling the flow from a fixed displacement pump to provide the desired amount of output flow when the system is in its working mode and to provide only a minimal amount of lubricating flow when the system is in its non-working mode.
In many applications, a fixed displacement hydraulic pump is mounted on mobile equipment and is driven by the same prime mover which provides the power for the piece of mobile equipment. The fixed displacement pump provides hydraulic fluid under pressure to various fluid actuated devices which are typically utilized only a small portion of the time that the vehicle prime mover, which is typically an internal combustion engine, is operating. It is well known that unless means are provided for disengaging a pump from the vehicle engine two undesirable effects occur. First, engine power is wasted resulting in less economical operation of the engine and, second, the hydraulic circuit becomes a source of significant noise, especially at high engine speeds.
One known solution to the above problems is the use of a large hydraulic pump having sufficient capacity to operate the desired fluid actuated devices when the pump is operated at or near the idling speed of the vehicle engine. In such a system, a clutch is typically provided so that when the vehicle is in motion, the operator may disengage the pump. Alternatively, it is known to use a centrifugal clutch in such a system to disengage the pump at high engine speeds. Such systems have operated successfully but are relatively expensive to manufacture.
Another solution to the above-identified problems is disclosed in U.S. Pat. No. 3,935,917 wherein a pump control system for a gear pump includes means for interrupting the flow of operating fluid to the pump during periods of time when the output of the pump is not required. Thus, theoretically, a pump such as that disclosed in U.S. Pat. No. 3,935,917, would operate only with any residual oil on the surface of the working parts during periods when the output of the pump was not required thereby reducing the load on the vehicle engine and the noise level associated with operating the system. In practice, when utilizing systems of the type disclosed in U.S. Pat. No. 3,935,917 it has been found necessary to place an orifice in the inlet valve to provide some flow through the pump for adequate lubrication of the pump components during periods when the output of the pump is not required. However, such systems still suffer several disadvantages. First, they cause a great deal of hydraulic noise during the brief period when the inlet valve is being closed; second, components have experienced inordinately short life spans; and third, such systems have been housed in relatively large packages thereby wasting space on the vehicle that could be more profitably allotted to other functions.